feat(vhd-cli/VhdAbstract): make derived values getters
feat(vhd-cli/VhdAbstract): make derived values getters

It makes them read-only, make sure they are always up-to-date with the header and avoid duplicating their logic.

lmcro/xo-web,lmcro/xo-web,vatesfr/xo-web,vatesfr/xo-web,lmcro/xo-web